1. Field of the Invention
The present invention relates to an acceleration sensor using piezoelectric ceramics.
2. Description of the Related Art
Various acceleration sensors using piezoelectric ceramics have been suggested. An example of such an acceleration sensor includes a detection element having a plurality of piezoelectric ceramic layers laminated together and a pair of retaining members that retain an end portion of the detection element in a longitudinal direction thereof at two principal surfaces of the end portion. This acceleration sensor generates a large amount of energy, which can be expressed as ½ of the product of charge sensitivity and voltage sensitivity, and functions as a high S/N ratio sensor, that is, a high sensitivity sensor.
FIG. 6 shows an example of an acceleration sensor disclosed in Japanese Unexamined Patent Application Publication No. 2003-337140 and Japanese Unexamined Patent Application Publication No. 2005-164505. The acceleration sensor 1 includes a detection element 2 and a pair of retaining members 10 and 11 that retain an end portion of the detection element 2 in a longitudinal direction thereof at two principal surfaces of the end portion. The detection element 2 is obtained by integrally firing a laminate of four piezoelectric ceramic layers 2a to 2d. The first layer 2a, the second layer 2b, the third layer 2c, and the fourth layer 2d are arranged in that order from one of the principal surfaces of the detection element 2 to the other one of the principal surfaces. An interlayer electrode 3 is provided at the approximate center of the detection element 2 in the thickness direction thereof, that is, between the second layer 2b and the third layer 2c. In addition, interlayer electrodes 4 and 5 are provided between the first layer 2a and the second layer 2b and between the third layer 2c and the fourth layer 2d, respectively. The detection element 2 includes principal-surface electrodes 6 and 7 on the principal surfaces thereof.
The first layer 2a and the fourth layer 2d of the piezoelectric ceramic layers have substantially the same thickness, and the second layer 2b and the third layer 2c of the piezoelectric ceramic layers have substantially the same thickness. The thickness of the second layer 2b and the third layer 2c is greater than the thickness of the first layer 2a and the fourth layer 2d. As shown by the arrows P in FIG. 6, the piezoelectric ceramic layers 2a to 2d are polarized in the thickness direction. The second layer 2b and the third layer 2c are polarized in the same direction, the first layer 2a and the second layer 2b are polarized in opposite directions, and the fourth layer 2d and the third layer 2c are polarized in opposite directions.
The end portion of the detection element 2 is retained by retaining portions 10a and 11a of the retaining members 10 and 11, respectively. Each of the interlayer electrodes 4 and 5 extends to an end surface of the end portion at one end thereof, and is electrically connected to an external electrode 8 provided continuously on end surfaces of the retaining members 10 and 11 and the end surface of the detection element 2. Each of the interlayer electrodes 4 and 5 extends to a location spaced part from a free end of the detection element 2 by a predetermined distance at the other end thereof.
The interlayer electrode 3 extends to a location corresponding to inner edges of the retaining portions 10a and 11a of the retaining members 10 and 11 at one end thereof, and to the free end of the detection element 2 at the other end thereof. Each of the principal-surface electrodes 6 and 7 extends to an intermediate location of the portion clamped between the retaining portions 10a and 11a of the retaining members 10 and 11 at one end thereof, and to the free end of the detection element 2 at the other end thereof. Extraction electrodes 12 and 13 are provided on inner surfaces of the retaining portions 10a and 11a, respectively, and are electrically connected to the principal-surface electrodes 6 and 7, respectively, with anisotropic conductive adhesive 14. As mentioned above, each of the principal-surface electrodes 6 and 7 extends to an intermediate location of the retaining portions 10a and 11a of the retaining members 10 and 11 at one end thereof. The reason for this is to provide sufficient opposing areas between the principal-surface electrodes 6 and 7 and the extraction electrodes 12 and 13 provided on the inner surfaces of the retaining portions 10a and 11a, respectively, thereby increasing the connection areas. The extraction electrodes 12 and 13 and the principal-surface electrodes 6 and 7 are not conductively connected to the external electrode 8. The extraction electrodes 12 and 13 extend continuously from the inner surfaces of the retaining portions 10a and 11a to inner surfaces of portions of the retaining members 10 and 11 at an end opposed to the retaining portions 10a and 11a. The extraction electrodes 12 and 13 are electrically connected to an external electrode 9 provided on end surfaces of the retaining members 10 and 11 and an end surface of an end member 15 at the end opposite to the retaining portions 10a and 11a. 
The detection element 2 includes a connection electrode 18 formed by vapor deposition or sputtering on a side surface at the free end thereof. The connection electrode 18 connects the interlayer electrode 3 and the principal-surface electrodes 6 and 7 to one another. The connection electrode 18 is provided in an area in which the interlayer electrodes 4 and 5 do not extend, and therefore is not connected to the interlayer electrodes 4 and 5. Instead of providing the connection electrode 18 on the side surface of the detection element 2, the connection electrode 18 may also be provided on an end surface of the detection element 2 at the free end thereof. When the connection electrode 18 is provided, an electrode 19 is also provided on side surfaces of the retaining members 10 and 11 and the end member 15. However, the electrode 19 may also be omitted.
As described above, the interlayer electrodes 4 and 5 are connected to the external electrode 8. In addition, the interlayer electrode 3 and the principal-surface electrodes 6 and 7 are connected to one another via the connection electrode 18, and are also connected to the external electrode 9 via the extraction electrodes 12 and 13 provided on the inner surfaces of the retaining members 10 and 11, respectively. Thus, as shown in FIG. 7, the four piezoelectric ceramic layers 2a to 2d are electrically connected in parallel between the external electrodes 8 and 9.
Referring to FIG. 6, when an acceleration G is applied in the direction shown by the arrow, the detection element 2 is deflected by inertia in a direction opposite to the direction in which the acceleration is applied. Accordingly, a compressive stress is applied to an upper half of the detection element 2 and a tensile stress is applied to a lower half of the detection element 2. Therefore, positive charges are generated in the interlayer electrode 3 and the principal-surface electrodes 6 and 7, and negative charges are generated in the interlayer electrodes 4 and 5. As a result, the negative charges are obtained from the external electrode 8 that is conductively connected to the interlayer electrodes 4 and 5, and the positive charges are obtained from the external electrode 9 that is conductively connected to the interlayer electrode 3 and the principal-surface electrodes 6 and 7.
In the acceleration sensor having the above-described structure, when a temperature change is externally applied, heat is transmitted to the detection element 2 through the retaining members 10 and 11. Accordingly, charges shown by the positive and negative signs in FIG. 8 are generated in the piezoelectric ceramic layers 2a to 2d by the pyroelectric effect. If the temperature change is evenly applied to the piezoelectric ceramic layers 2a to 2d, the charges generated by the pyroelectric effect cancel each other between the first layer 2a and the fourth layer 2d and between the second layer 2b and the third layer 2c. Therefore, no output is obtained. However, if the temperature is increased locally, different temperature changes occur in the piezoelectric ceramic layers 2a to 2d. Therefore, the charges generated by the pyroelectric effect do not cancel each other out between the first layer 2a and the fourth layer 2d or between the second layer 2b and the third layer 2c. As a result, an unwanted output (thermal fluctuation noise) is output from the acceleration sensor 1. For example, if the temperature of the retaining member 10 is greater than that of the retaining member 11, heat is transmitted from the retaining portion 10a of the retaining member 10 to the upper surface of the end portion of the detection element 2. Therefore, a temperature difference is generated between the upper surface and the lower surface of the detection element 2, and an unwanted output is generated by the pyroelectric effect.
The cause of the generation of the unwanted output by the pyroelectric effect will be described in more detail. The piezoelectric ceramic layers 2a to 2d included in the detection element 2 are polarized in the directions shown by the arrows P. As shown by the knurling pattern in FIG. 8, polarized areas of the piezoelectric ceramic layers 2a to 2d correspond to areas of the piezoelectric ceramic layers between the electrodes facing one another. This is because the interlayer electrodes 3, 4, and 5 and the principal-surface electrodes 6 and 7 are also used as polarizing electrodes. In this structure, pyroelectric charges are generated in polarized areas (denoted by S in FIG. 8) of the first layer 2a and the fourth layer 2d within a retaining area where the end portion is retained by the retaining members 10 and 11. This is because no pyroelectric charge is generated in non-polarized areas or in areas between electrodes having the same potential. More specifically, portions of the principal-surface electrodes 6 and 7 extend into the areas where the portions of the principal-surface electrodes 6 and 7 face the retaining portions 10a and 11a of the retaining members 10 and 11, respectively. Therefore, pyroelectric charges are generated in the areas S between the extending portions of the principal-surface electrodes 6 and 7 and the interlayer electrodes 4 and 5, and are extracted as an unwanted output.